1. Field of the Invention
The present invention relates to a catalyst layer-supporting substrate, a method of producing the catalyst layer-supporting substrate and a fuel cell.
2. Description of the Related Art
Polymer electrolyte fuel cells, in particular methanol-type polymer electrolyte fuel cells that use methanol solution as fuel and allow operation at low temperature and reduction in size and weight, have been studied eagerly as a power source of, for example, mobile devices. However, noble metal have been used as the main component of the catalyst materials for fuel cell electrodes, and thus, such electrodes are higher in cost and yet to be at the level allowing wide-spread use. There is a need for a technology to provide high fuel cell performance, by using a smaller amount of the noble metal catalyst material.
In the conventional method of production of electrodes for fuel cells, so-called catalyst layer-supporting substrates, slurry is prepared from a catalyst material, a proton conductor, a solvent and the like by mixing them. The slurry is then coated on a substrate. However, the method has a problem that the loss of the catalyst in the process is very large, approximately 30%.
Production methods by sputtering or vapor deposition have also been studied as a process in which the catalyst loss is decreased.
For example, WO2002/073722 reports that a catalyst layer is formed on a substrate by sputtering using a catalyst material and then a layer of particulate carbon is formed thereon. However, when a catalyst layer is prepared by the method, fine particles of the catalyst material aggregate significantly, causing a problem of insufficient utilization efficiency of the catalyst material.
Also reported is a method of forming a multilayer structure by sputtering using a catalyst material followed by formation of a Fe-containing metal layer by sputtering and then forming a porous structure in the catalyst layer by removing the Fe-containing metal layer in a pore-forming process (JP-A 2007-507328). Also reported is a method which comprises forming a catalyst layer and forming pores in the catalyst layer in the following pore-forming process (U.S. Pat. No. 4,126,934). The catalyst layer is formed by making an alloy of an easily soluble metal, so-called pore-forming metal, and a catalyst material, or sputtering or vapor deposition of the mixture thereof. However, both of the catalyst layers are still unsatisfactory in utilization efficiency and durability of the catalyst material and demand further improvement.